Propane heat pump key to efficiency?

Researchers at the Fraunhofer Institute for Solar Energy Systems are developing a heat pump that uses propane. They have found a way to mitigate the flammability impact and make it a feasible solution…

The newly-developed heat pump takes its inspiration from the branching limbs of a tree.

Heat pumps use environmental energy to provide us with heat. However, they generally require synthetic refrigerants, which contain environmentally harmful fluorinated greenhouse gases (F-gases). According to Phys.Org, Fraunhofer researchers have now contributed to the development of a heat pump that uses propane instead. The pump is both more climate-friendly and more efficient.

"Heating and hot water account for around 40% of Germany's final energy consumption. Burning high-quality fossil fuels such as natural gas or crude oil not only makes little sense energetically, it also harms the climate. Each unit of electrical energy required to operate a heat pump, derived often from renewable resources, generates three to five units of CO2-neutral heat energy. This makes heat pumps an important element in implementing Germany's transition to a sustainable energy system," says D0. Marek Miara, who coordinates work on heat pumps at the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg.

For the most part, refrigerants are composed of a mixture of synthetic substances containing environmentally harmful, fluorinated greenhouse gases (F-gases). In June 2014, the European Commission announced that F-gases are to be phased out of the market. One environmentally friendly, natural alternative to synthetic refrigerants is propane, which is already gaining in popularity in air conditioning and refrigeration systems. But its use in heat pumps is still relatively new.

Because even though propane has excellent thermodynamic properties, it is highly flammable, and this poses a challenge when used in a heat cycle.

"If you want to use propane, you have to keep the volume of refrigerant as low as possible to minimise the risks involved," says Dr Lena Schnabel, who heads the department for heating and cooling technologies at Fraunhofer ISE.

The solution

The solution of the ISE researchers, along with their European research partners, is to employ highly compact, brazed, finned heat exchangers that function well with small volumes of liquid. The thermal energy is transferred from one flowing substance to the other via heat exchangers. These are composed of numerous parallel channels containing the circulating refrigerant, which either absorb heat (known as ‘vaporisers’) or radiate it (‘condensers’).

"The liquid should completely vaporize or recondense over the running length. To guarantee they operate efficiently, the vapour-liquid ratio must be identical in all the channels. Generally, that's not easy to achieve, and it becomes especially tricky if you're also trying to limit the volume of refrigerant."

To solve the problem, Schnabel and her team developed a distributor with a bionic structure. "Conventional Venturi distributors look like a pile of spaghetti made of many thin tubes that merge where they meet the vaporiser. Our distributor is different: it has a continuously branching structure like the branches and twigs of a tree, which ensure even distribution of the refrigerant into the individual evaporator channels, even with a small volume of refrigerant." This structure allows optimal use of the entire surface of the heat exchanger, which improves efficiency.

To reduce the risk of explosion when compressing the propane, Schnabel and her team used a specialised compressor in which all ignition sources are encapsulated. They took great care connecting the individual components of the pump to prevent propane from escaping.

"We are currently modifying the technical design of the heat pump, testing the long-term behaviour of its components, and developing sustainable safety strategies," says Schnabel.